In recent years there has been an increased understanding of the role of activity-dependent plasticity in promoting functional recovery following neurologic injury. There is also an emerging appreciation for the role of spinal mechanisms in control of behaviors such as walking. Research indicates that activity-dependent adaptive changes in spinal cord circuitry may play an important part in the recovery of walking function in individuals with spinal cord injury (SCI), perhaps long after the stage of spontaneous recovery. Two technologies that appear effective in promoting functional recovery are body weight support (BWS) and functional electrical stimulation (FES). In preliminary studies, improved overground walking speed, leg strength and endurance was observed in subjects with SCI who trained using an approach combining these interventions. This proposal will assess, in individuals with chronic incomplete SCI, responses to a BWS-assisted treadmill training regimen under one of three training conditions: 1) passive (manual)-assisted stepping, 2) active (FES)-assisted stepping, or 3) non-assisted (voluntary) stepping. Each condition has distinct advantages. Manually-assisted stepping allows assistance to be graded to individual needs; FES-assisted stepping takes advantage of spinal reflex circuitry and may contribute to beneficial changes in spinal cord neurophysiology; and voluntary stepping may encourage the greatest voluntary effort. These investigations will determine which condition best promotes efficient overground walking, and will examine changes in spinal cord neurophysiology, strength and cardiorespiratory capacity associated with the three forms of training. Performance-based measures will include: computerized kinematic analysis to assess lower limb coordination and key parameters of gait (speed, stride length and step rhythm), electromyographic assessment of muscle recruitment and work capacity evaluation to assess changes in strength and gait efficiency. Neurophysiologic techniques (e.g., reflex testing and transcranial magnetic stimulation) will be used to assess supraspinal, spinal and peripheral control of mechanisms that contribute to walking, to voluntary movement and to reflex activation. The proposed experiments will test the hypotheses that: 1) walking performance in subjects with SCI improve to a greater extent when active, as opposed to passive or no assistance is given to advance the limb, 2) post-training spinal cord reflex activity will be more similar to normal values in those subjects who participate in the FES-assisted group, and 3) electromyographic (EMG) activity observed during walking will be more robust than EMG observed during voluntary movement. These studies will allow us to compare the efficacy of rehabilitation strategies aimed at improving functional mobility in individuals with SCI. In addition, they will provide information about neural and cardiorespiratory bases of these functional changes.